The present invention relates generally to spark plugs and methods of producing the spark plugs.
A spark plug for internal combustion engine usually has a center electrode and another ground electrode, which upon supplying of an electrical potential generate a spark for igniting fuel in an internal combustion engine. Since the ignition is performed many thousand times due to an electrical discharge at the electrodes, the electrodes are subjected to erosion which reduces their service life. In order to increase the service life, it was proposed to apply reinforcing materials on the facing surfaces of the electrodes by the range of different techniques. Some techniques require sophisticated equipment and control, following shaping electrode tips into final configuration (attaching by welding small reinforce metal pieces to electrode ends, inserting metal wire in a hole of central electrode tip, plasma welding technique, etc.). Other processes apply a layer of reinforcing material by sputtering or coating methods which can not provide reliable bonding between the electrode substrate and the layer material at high temperature spark plug operating conditions and could result in failure of a spark plug. Most of techniques use precious metals as a reinforcing material, which have extremely high cost. Some spark plugs of the above mentioned general type are disclosed in U.S. Pat. Nos. 3,673,452, 4,810,220, 4,881,913, 5,179,313, 5,456,624; 5,779,842; etc.
Accordingly, it is an object of the present invention to provide a spark plug and a method of producing the same which avoids the disadvantages of the prior art.
In keeping with these objects and with others which will become apparent hereinafter, one feature of present invention resides, briefly stated, in a spark plug which includes at least two electrodes producing a spark by an electrical discharge and having ends facing toward one another, wherein an additional layer of the material, in particular tungsten, is applied on the ends of the electrodes facing toward one another by electro-spark deposition.
When the spark plugs are formed in accordance with the present invention, their erosion resistance and therefore their service life is substantially increased. The layer produced on the facing surfaces of the electrodes is finally shaped and its bonding to the electrodes is strong due to the diffusion. The diffusion transient zone has linear coefficient of expansion between such of the erosion resistant layer and an electrode material. Thereby, heat stresses that occur on a border between layer and electrode body during the spark plug operation will relieve.
In accordance with another feature of present invention, a method of producing the spark plug is proposed, in accordance with which at least two electrodes are made, and then an additional layer is applied on the end of each electrode facing the other electrodes, the layer composed of tungsten and applied by electro-spark deposition, so that a reliable diffusion bonding is provided between the coating and the spark plug electrode material and heat stresses that occur on a border between the layer and the electrode body during the spark plug operation will be relieved.
The novel features which are considered as characteristic for the present invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.